The present invention relates to a tire for a vehicle wheel in which at least one of the beads comprises a seat having a generatrix, the axially inner end of which lies on a circle of diameter greater than the diameter of the circle on which the axially outer end is located. This type of design is particularly suited to the new generations of tires which can be used, within certain limits, under low-pressure, or even zero- or virtually zero-pressure conditions, and which have a lesser risk of separation of the tire from the rim on which it is mounted. This concept is frequently designated by the expression “extended mobility”.
For a long time, tire manufacturers have been trying to develop a tire which does not create any source of risk or potential danger in the event of an abnormal drop in, or even total loss of, pressure. One of the difficulties encountered relates to travelling with a flat tire or at very low pressure, because, when travelling at very low pressure, or even at zero pressure, with conventional tires, the beads are at great risk of separating from the periphery of the rim against which they were held by the pressure.
Numerous solutions have been tested in order to overcome these disadvantages. Frequently, these solutions cause additional difficulties in mounting and demounting the tire on/from the rim.
Furthermore, the clamping function of the tire on the rim is an essential function for ensuring the qualities of the tire in operation, because it directly or indirectly affects many aspects such as mounting (sometimes referred to as “clipping”) or fastening of the tire, the air-tightness of the tire, non-rotation on rim, etc. These functions are all important and require specific characteristics and rigorous manufacture of the products, in particular if high quality standards are desired. Now, the rims and tires frequently, for a given dimension, have slightly different actual dimensions, mainly due to the manufacturing tolerances. These variations in dimensions complicate compliance with the different functions listed above.
To fulfill these functions, two broad types of solution are used industrially. First of all, for traditional tires, the bead wire simultaneously performs all these functions.
More recently, for several types of products manufactured by the Applicant, the conventional bead wire has been replaced by an anchoring zone comprising in particular arrangements of circumferential cords cooperating with the carcass-type reinforcement structure via an anchoring or bonding mix. In this case too, the anchoring zone performs all the functions set forth above.
However, in both these cases, it is difficult to optimise certain parameters because, very often, an improvement in one parameter causes another to deteriorate. There are thus certain limits to making such compromises between a gain on one hand and a loss on another, since it is often difficult to tolerate poorer performance for certain aspects.
EP 0 582 196 discloses a tire comprising a tread extended by two sidewalls and two beads and also a carcass anchored in the two beads to an annular reinforcement. The carcass is formed of cords in an adjacent arrangement which are aligned circumferentially and are in contact with at least one layer of bonding rubber of very high elasticity modulus in the hooking zone of the bead comprising the annular reinforcement. In this tire, the annular reinforcement of the hooking zone of the bead is formed of stacks of circumferential cords with interposition of a layer of bonding rubber of very high elasticity modulus between the reinforcement cords of the carcass and these stacks. This embodiment is intended for tires of conventional type, with the beads being held against the rim hook due to the inflation pressure of the tire. In this type of arrangement, there is a predominance of forces in the lateral or axial direction, which induces major compressive forces which act substantially axially from the walls towards the centre of said bead. These forces increase according to the inflation pressure. The increase in pressure tends to make the bead slide against the hook, radially towards the outside. The stresses induced radially towards the inside, against the seat of the rim, decrease with the increase in pressure, or with any increase in the tension of the carcass-type reinforcement structure.
It will furthermore be noted that the stacks of cords are aligned in a direction substantially parallel to the orientation of the profile of the rim hook against which the bead bears. The profile of the bead of this type of tire is relatively narrow and elongated; the anchoring is distributed over the major part of the height and width of the bead. The passage of the carcass into the bead is generally substantially central relative to the walls of said bead. Furthermore, when it is a relatively narrow bead subject to predominantly axial forces, neither the inflation pressure nor the tension induced in the carcass permits generation of large moments or torques, which tend to make the bead pivot or turn on itself.
With such a type of tire, if the pressure drops and the vehicle continues to travel, holding of the tire on the rim is no longer ensured, and in the majority of cases it rolls off the rim.
EP 0 673 324 describes a rolling assembly comprising at least one tire with a radial carcass reinforcement which is anchored within each bead and a rim of specific shaping. This rim comprises a first seat with a generatrix such that the axially outer end of said generatrix is distant from the axis of rotation by a length less than the distance between its axially inner end, and is defined axially to the outside by a protrusion or rim flange. The tire comprises bead seats suitable for mounting on this rim. The type of tire/rim interface proposed in this document has many advantages compared with the solutions already known, in particular with regard to the ease of mounting/demounting, while making it possible to travel a certain distance despite a drop in pressure.
EP 0 748 287 describes a solution which permits initial optimisation of the basic technology described in EP 0 673 324 referred to above. This is a tire, at least one bead of which has a structure which makes it possible to modify the clamping of said bead according to the tension of the carcass reinforcement and in particular reinforcement thereof when the inflation pressure increases to its rated value. The document thus proposes using a bead with anchoring of the end of the carcass by turning it up about the base of the bead wire, via the axially and radially inner sides relative to the bead wire. The bead also comprises, adjacent to the bead wire and axially to the outside thereof, a profiled element of rubber mix of relatively high hardness against which the bead wire can exert a compressive force when the tension of the carcass reinforcement increases. This compressive force creates self-clamping of the toe of the bead on the mounting rim. The tension of the carcass therefore involves displacement of the bead wire towards the outside, so that the latter generates said compressive force. In such a configuration, the presence of a bead wire of conventional type and the turning-up of the carcass beneath the latter are presented as being indispensable for generating the compressive force. This restricts the other types of arrangement which can be considered
Moreover, EP 0 922 592 describes two embodiments with the carcass anchored by turning it up axially towards the outside. The first embodiment proposes anchoring of the carcass in the bead by turning it up radially towards the outside of the end of the carcass. The upturn is surrounded on either side by two radially superposed layers of metal wires arranged axially side by side and covering substantially all the axial portion along the seat of the bead. The layers are arranged so as to be parallel to the seat. The types of wires and the corresponding dimensions are very precise. The second solution proposed in this document relates to bead seats with different diameters. The carcass is also secured differently from the first solution. First of all, the carcass is subdivided into two portions which are radially separated at the level of the bead. Each portion is adjoined by a layer of wires which is arranged radially, each layer being arranged radially to the outside against each of the carcass portions. The radially outer carcass portion and the layer of wires radially to the inside are separated by an insert of the elastomer of high hardness type provided in the bead. This insert axially lines the central portion of the bead and rises radially towards the outside and axially towards the inside, beyond the radial limit of the presence of the metal wires.
The two examples of solutions in EP 0 922 592 have several disadvantages. Thus, the securing of the carcass proposed in this document requires the presence of an upturn axially towards the outside of the end portion of the carcass. Furthermore, the superposed layers of wires are arranged radially close to the seat of the bead, for a good part at a radial position closer to the axis of rotation than the upper portion of the flange on which the bead bears. Unless highly extensible wires are used, it is difficult to mount/demount the tire, due to the unfavourable radial position of the wires. It will also be noted that the stacks are oriented substantially parallel to the profile of the seat against which the bead bears. According to the second solution, the carcass is subdivided into two portions and an insert of high hardness is necessary to separate on one hand the layers of wires and on the other hand the two carcass portions. However, the carcass is not anchored in the insert. The form of the insert described is limitative.
Document WO 01/39999 describes an extended-mobility tire, each of the beads of which comprises an inverted seat, an anchoring zone, a bearing zone and a transition zone. Each of the zones taken in isolation and also all the zones together to some extent form an internal bead capable of effecting relative movements, such as, for example, of the angular or rotational type, relative to another zone, or relative to a virtual centre of pressure CP, or relative to the seat of the rim, etc.
Preferably, said bearing zone is substantially elongated. It is extended, for example, substantially along the seat of the bead. The transfer of forces upon rotation of the bottom zone of the axially inner portion towards the axially outer portion is thus possible, while maintaining bearing pressure against at least one portion of the seat of the bead. The transfer of the forces creates self-clamping of the toe of the bead against the rim.
The present invention therefore proposes to overcome the various disadvantages inherent in the solutions set forth above. It proposes in particular a solution aimed at improving the dynamic stability of the anchoring zone.
To do this, it provides a tire for a vehicle wheel, comprising:    two sidewalls spaced apart axially from each other, joined at their radially outer portions by a crown zone provided on its radially outer portion with a circumferential tread;    beads, arranged radially to the inside of each of the sidewalls, each bead comprising a seat and an outer flange which are intended to come into contact with a suitable rim;    a reinforcement structure extending substantially radially from each of the beads, along the sidewalls, towards the crown zone;    at least one of said beads comprising:    a bead seat comprising a generatrix the axially inner end of which is located on a circle of diameter greater than the diameter of the circle on which the axially outer end is located;    an anchoring zone for the reinforcement structure in said bead, comprising an arrangement of circumferential cords arranged substantially adjacent to a portion of the reinforcement structure and comprising at least two stacks distributed on either side of the reinforcement structure, a bonding mix being arranged between the circumferential cords and the reinforcement structure, said anchoring zone being arranged in said bead in such a manner that, at normal pressure, the forces of the reinforcement structure are distributed substantially homogeneously on either side of said structure, in said anchoring zone, said reinforcement structure being arranged so as to obtain a circumferential distribution of the cords on either side of at least one of said stacks;    a bearing zone for said bead extending substantially along the seat of the latter;    said tire also comprising an external lateral zone arranged in the zone of the bead provided to be arranged between the rim hook and the anchoring zone, said zone being filled by a rubber mix of substantially high modulus.
Such a configuration makes it possible to achieve an optimum distribution of forces at the level of the anchoring zone, in particular in the arrangement of circumferential cords. Major differences in both the nature and the level of stresses to which the various cords in the arrangement are subjected are, for example, avoided, some cords for example being subjected to tensile loads, while others are subjected to compressive loads.
This more uniform distribution of the stresses is particularly advantageous for certain types of tires, in particular those having a very high sidewall, such as for vehicles of “SUV” type.
Furthermore, the specific arrangement of the cords of the carcass-type reinforcement structure so as to obtain more than one axial position of these cords in the anchoring zone contributes to distributing the forces at different locations. In the examples illustrated, there can be seen preferably two axially spaced positions in which the axially inner and axially outer portions of carcass-type reinforcement structure can be seen. In this manner, a supplementary adjustment means is obtained which makes it possible to produce arrangements in which the aim is as far as possible towards equilibrium or uniformity of the stresses. The benefits obtained in terms of durability and reliability may be considerable.
The cords of at least one stack of circumferential cords are consequently “surrounded”, since some cords of the carcass-type reinforcement structure are axially to the outside relative to said stack, whereas others are axially to the inside relative to said stack, preferably alternating regularly along the circumference of the tire. This may be an alternation at each cord, at every two cords, three cords, or even more.
Advantageously, the external lateral zone is provided in the axially outer portion of the bead and extends between the portion adjacent to the rim hook and the anchoring zone. Advantageously, said zone cooperates with the anchoring zone, which permits a better mechanical action between said anchoring zone and the portion of the bead adjacent to the rim hook.
This zone makes it possible to increase the clamping pressure, in particular in the region of the rim hook. Thanks to the limited deformability of the zone, it makes it possible to limit the tendency of the bead to slip radially outwards beyond the rim hook. It furthermore contributes, on the one hand, to the inhibition of any tendency to generate a rotational moment and, on the other, to establish dynamic stability, such as for example when cornering or on exposure to major lateral stresses.